Holograms have been used for display of three-dimensional images, measurement, optical elements and other applications. Mass duplication of holograms on a commercial scale has hitherto been carried out as follows.
Calculation is first carried out by means of a computer so that holograms can surely have necessary properties. The results of calculation are written by means of an electron beam exposure system to prepare a first original plate for a hologram.
Separately, a recording material for hologram formation is coated onto a glass substrate to form a recording material layer for hologram formation, thereby providing a dry plate for photographing a hologram. The original plate for a hologram and the dry plate for photographing a hologram are put on top of each other in such a manner that the original plate for a hologram on its hologram face comes into contact with the recording material layer for hologram formation in the dry plate for photographing a hologram. This is followed by exposure of the assembly to a laser beam through the original plate for a hologram.
The exposure causes interference of first-order diffracted light, which has been diffracted by a diffraction grating of the hologram in the original plate for a hologram, with zero-order diffracted light, which has traveled substantially straight, within the recording material layer for hologram formation. This creates fringes, and polymerization occurs within the recording material layer for hologram formation according to the fringes.
After the exposure, necessary development is carried out to duplicate the hologram in the original plate onto the recording material layer for hologram formation, coated on the glass substrate.
The duplicated hologram is in some cases used as such. In other cases, this duplicated hologram is used as an original plate for duplication. Specifically, a dry plate for photographing a hologram as described above is provided, and the original plate for duplication on its hologram side is disposed so as to face the recording material layer for hologram formation in the dry plate for photographing a hologram, followed by exposure in the same manner as described above. In this way, holograms are mass produced.
A hologram, wherein illumination light for reproduction is admitted from one side of the hologram and the diffracted light goes out from the opposite side of the hologram, is known as a transmission hologram. In photographing or duplicating a transmission hologram, an object light and a reference light are admitted from the front side of the dry plate for photographing a hologram. The object light and the reference light, which have reached the back side of the dry plate, are reflected from the back side of the dry plate to cause halation. Due to the halation, unnecessary interference fringes are recorded, resulting in deteriorated reproduction properties of the hologram.
For this reason, in general, a layer for absorbing a transmitted laser beam (an antihalation layer) has hitherto been provided on the back surface of the dry plate for photographing a transmission hologram to reduce the occurrence of undesired interference fringes attributable to the reflection of the laser beam from the back surface of the dry plate. The antihalation layer is generally formed by coating a resin solution containing a dye dissolved therein and drying the coating. An example of the dye-containing resin solution is a solution prepared by admixing a suitable amount of a dye into a methyl ethyl ketone solution of polyvinyl butyral (PVB).
The above conventional method for the formation of an antihalation layer by coating, however, involves the following problems. Specifically, the resin solution should be coated followed by drying of the coating. This, for example, complicates the process and deteriorates the productivity. At the same time, the coating solution migrates to the opposite side of the dry plate to soil the surface of recording material for hologram formation. Further, for applications where a high level of cleanness is required, for example, the step of wiping off and completely removing the antihalation layer with a solvent should be additionally provided after recording the hologram.
In order to solve the above problems of the prior art, Japanese Patent Laid-Open No. 54539/1997 proposes a self-adhesive film for hologram formation, a dry plate for photographing a hologram, and a method for image formation using the same, wherein the antihalation layer is in a dry film form which can significantly simplify the production process as compared with the conventional production process and, after recording a hologram, can be completely removed by a simple step.
Further, in the photographing or duplication of reflection holograms, reference light is admitted through the surface of the dry plate for photographing a hologram, while object light (diffracted light from an object or an original plate for a hologram) is admitted through the back surface. In this case, the object light or the reference light, which had reached the air/dry plate interface in the dry plate, is subjected to interface reflection to cause halation. As a result, unnecessary interference fringes are recorded, resulting in deteriorated hologram reproduction properties. For this reason, in general, an antireflection glass or a color glass (ND (=neutral density) glass) for interface reflection prevention purposes is installed on the surface of the air/dry plate in the dry plate for a reflection hologram.
The antireflection glass or the color glass is stacked on the air/dry plate interface through an optical liquid for intimate contact. In this case, the optical liquid for intimate contact cannot easily form an evenly thick coating due to flow, vibration, inclusion of air or the like. In particular, when the amount of the liquid for intimate contact is large, the time elapsed until, after coating, the flow ceases and becomes stable, is long, resulting in poor workability. In order to overcome this drawback, for example, Japanese Patent Laid-Open Nos. 281883/1993, 281583/1995, and 114329/1995 propose methods wherein a flexible color film or antireflection film is used instead of the glass to prepare a dry plate for a hologram without use of the liquid for intimate contact.
In the above conventional methods for hologram formation, however, the use of a pressure-sensitive adhesive for bringing the antihalation layer into a dry film form poses a problem that the refractive index of conventional pressure-sensitive adhesives is about 1.4 to 1.46 which is greatly different from the refractive index of the transparent substrate supporting the recording material layer for hologram formation, about 1.52, leading to reflection at the interface of the transparent substrate and the pressure-sensitive adhesive layer and consequently unsatisfactory prevention of halation by the pressure-sensitive adhesive layer.
The present inventors have further found that, in the case of hologram formation using the conventional antihalation layer formed of a pressure-sensitive adhesive, the reflected light is rapidly increased when the angle of incidence of the hologram recording light exceeds 50 degrees, particularly 55 degrees, that is, when the hologram recording light is admitted at a low angle as viewed from the interfacial boundary.